1. Electric Motors Types of Electric Motors
North Seattle Community College HVAC Program
Instructor – Mark T. Weber, M.Ed., CMHE
Elec motors - 2

2. Objectives After studying this unit, you should be able to:
Describe the different types of open single-phase motors used to drive fans, compressors, and pumps
Describe the applications of the various types of motors
State which motors have high starting torque

3. Objectives (cont’d.)
List the components that cause a motor to have a higher starting torque
Describe a multispeed permanent split-capacitor motor and indicate how the different speeds are obtained
Explain the operation of a three-phase motor
Describe a motor used for a hermetic compressor

4. Objectives (cont’d.)
Explain the motor terminal connections in various compressors
Describe the different types of compressors that use hermetic motors
Describe the use of variable-speed motors

5. Uses of Electric Motors
Used to turn fans, pumps, compressors
Facilitate the circulation of air, water, refrigerant, and other fluids
Motors are designed for particular applications
The correct motor must always be used
Most motors operate on similar principles

6. Fans are used to move air
Pumps are used to move liquids

7. Parts of an Electric Motor
Stator with motor windings: stationary portion of the motor
Rotor: rotating portion of the motor
Bearings: allow free rotation of the shaft
End bells: supports bearings and/or shaft
Housing: holds all motor components together and facilitates motor mounting

8. Parts of an Electric Motor (cont’d.)
Figure 17–3 Individual electric motor parts

9. Electric Motors and Magnetism
Electricity and magnetism are used to create rotation
Stator has insulated windings called run windings
Rotor may be constructed of bars
Squirrel cage rotor; positioned between the run windings
Rotor turns within the magnetic field

10. Figure 17–4 Poles (north and south) on a rotating magnet will line up with the opposite poles on a stationary magnet

11. Determining Motor Speed
As the number of poles increases, the motor speed decreases
Motor Speed (rpm) = Frequency x 120 ÷ # of poles
In the United States, the frequency is 60 Hz
For example, a two-pole motor will turn at a speed of 60 x 120 ÷ 2 = 7200 ÷ 2 = 3600 rpm

12. Determining Motor Speed (cont’d.)
The motor will turn at a speed that is lower than the calculated value
Slip = difference between calculated and actual motor speed

13. Start Windings Enables the motor to start and in the right direction
Start winding has higher resistance than the run winding
Wound with more turns
Wound with smaller diameter wire
Removed from the active circuit once the motor starts

14. Starting and Running Characteristics
List of characteristics:
Refrigeration compressors have high starting torque
Starting torque: twisting force that starts the motor
Locked Rotor Amperage (LRA)
Full Load Amperage (FLA)
Rated Load Amperage (RLA)
Motor may start with unequal pressures across it

15. Electrical Power Supplies
Residences are furnished with single-phase power
Houses can be supplied power from the transformer
Power feeds into circuit breaker panel or fuse box

16. Electrical Power Supplies (cont’d.)
Circuit breakers protect each individual circuit
Power is distributed throughout the house
Typical residential panels provide 115 and 230 volts
Commercial and industrial facilities require three-phase power

17. Figure 17–11 A wiring diagram of a main circuit breaker panel for a typical residence

18. Single-Phase Open Motors
Residential motors operate at 115, 208, or 230V
Commercial motors operate at voltages up to 460V
Some motors are designed to operate at one of two different voltage (dual voltage motors)
Dual voltage motors are wired differently for each voltage
Some motors have reversible rotations

19. Figure 17–14 The wiring diagram of a dual-voltage motor
Figure 17–14 The wiring diagram of a dual-voltage motor. This motor is designed to operate at either 115 V or 230 V, depending on how the motor is wired in the field (A) A 230-V wiring diagram (B) A 115-V wiring diagram

20. Split-Phase Motors Two separate motor windings Good running efficiency
Medium amount of starting torque
Speed typically ranges from 1800 – 3600 rpm
Motor speed is determined by the number of poles
Slip is the difference between the calculated and actual motor speeds

21. Split-Phase Motors (cont’d.)
Figure 17–16 Diagram of the start and run windings

22. The Centrifugal Switch
Commonly used on open motors to de-energize the start winding
Opens its contacts when the motor reaches about 75% of its rated speed
When the contacts open and close, a spark is created (arcing)
Not used in a refrigerant atmosphere

23. The Electronic Relay
Used to open the start windings after the motor has started
Solid-state device designed to open the start winding circuit when the design speed has been obtained

24. Capacitor-Start Motors
Split phase motor with start and run windings
Start capacitor assists the motor starting by increasing the starting torque
Start capacitor is wired in series with the motor’s start winding
Start capacitor is removed from the circuit when the start winding is removed
Start capacitor increases the phase angle

25. Capacitor-Start Motors (cont’d.)
Figure 17–22 Wiring diagram of a capacitor-start motor

26. Capacitor-Start, Capacitor-Run Motors
Most efficient single-phase motor
Often used with belt-driven fans and blowers
Run capacitor improves running efficiency
Run capacitor is in the circuit whenever the motor is energized
Start and run capacitors are wired in parallel
Motor amperage will rise if run capacitor goes bad

27. Capacitor-Start, Capacitor-Run Motors (cont’d.)
Figure 17–23 Wiring diagram of a capacitor-start, capacitor-run motor. The start capacitor is in the circuit only during motor start-up, whereas the run capacitor is in the circuit whenever the motor is energized

28. Permanent Split Capacitor (PSC) Motors
Simplest split-phase motor
Only a run capacitor is used
Low starting torque and good running efficiency
Can be single or multispeed motors, and multispeeds have leads for each speed
As resistance decreases, motor speed increases
As resistance increases, motor speed decreases

29. Permanent Split Capacitor (PSC) Motors (cont’d.)
Figure 17–27 This diagram shows how the windings of a three speed PSC motor are configured. As the winding resistance increases, the motor speed decreases

30. Shaded-Pole Motors Very low starting torque
Not as efficient as the PSC motor
A portion of the run winding is shaded to provide the imbalance in magnetic field that allows the motor to start
Heavy copper wire or bands are used to shade the run winding
Manufactured in the fractional horsepower range

31. Figure 17–29 Wiring diagram of a shaded-pole motor

32. Three-Phase Motors Normally used on commercial applications
Must have a three-phase power supply
Has no start winding or capacitors
Very high starting torque
Rotation of motor can be changed by switching any two power legs

33. Three-Phase Motor (cont’d.)
Figure 17–32 (A) Diagram of a three-phase power supply
(B) Diagram of a typical, single-speed, three-phase motor

34. Single-Phase Hermetic Motors
Hermetically sealed from outside air
Similar to single-phase motors
Use relays to remove start winding
They do not use centrifugal switches
Use run capacitors for increased efficiency
Designed to operate in a refrigerant atmosphere
Motor terminals identified as common, start & run

35. The Potential Relay Used on motors requiring high starting torque
Coil with very high resistance
Normally closed contacts
Relay operates on the induced voltage across the start winding
The contacts open when the induced voltage rises; when the induced voltage drops, the relay contacts close

36. (A)
(B)
(C)
Figure (A) A potential relay with its packaging box (B) Internals of a potential relay showing the coil and contacts (C) Diagram illustrating the higher induced voltage that is measured across the start winding in a typical motor

37. The Current Relay Used on fractional horsepower motors
Used with fixed-orifice metering devices
Low resistance coil in series with the run winding
Normally open contacts in series with start winding
Upon startup, only the run winding is energized

38. The Current Relay (cont’d.)
The motor draws locked rotor amperage
The increased amperage closes the relay contacts
The start winding is energized and the motor starts
The amperage drops and the relay contacts open

39. Figure 17–39 Wiring diagram of a current magnetic relay
Figure 17–39 Wiring diagram of a current magnetic relay. The “L” indicates line voltage, the “M” refers the main, or run, winding, and the “S” refers to the start winding. The coil is connected between the
“L” and “M” terminals, whereas the relay contacts are connected between the “L” and “S” terminals

40. Positive Temperature Coefficient Resistors (PTCRs)
Compressor starting devices that are resistors that vary their resistance when their surrounding temperature changes
Have a low resistance over a wide temperature range
Used often in the HVACR industry in place of current and potential relays

41. Two-Speed Compressor Motors
Used to control capacity of compressors
Speed changes made by wiring changes
The thermostat controls the wiring changes
Two compressors in one housing
One motor turns at 1800 rpm; the other at 3600
Two-speed compressors have more than three motor terminals

42. Special Application Motors
Facts:
Some single-speed motors have:
More than three motor terminals
Auxiliary compressor windings to increase the motor efficiency
Have winding thermostats wired through the compressor shell
Three-phase motors have one thermostat for each winding, wired in series

43. Three-Phase Compressor Motors
Used in large commercial/industrial applications
Normally have three motor terminals
No capacitors are required
Same resistance across each winding
High starting torque
Some larger three-phase compressor motors operate as dual voltage device

44. Variable Speed Motors Motor speed decreases during low load conditions
Voltage/frequency determine motor speed
New motors controlled by electronic circuits
Variable speed DC motors; ECM DC motors
Can ramp up/down to reduce motor wear
AC current can be converted to DC using rectifiers

45. DC Converters (Rectifiers)
Phase-controlled rectifier
Converts ac power to dc power
Uses silicon controlled rectifiers and transistors
Capacitors smooth out the dc voltage

46. Rectifiers (cont’d.) Diode bridge rectifier
Does not regulate the dc voltage
Diodes are not controllable
Voltage and frequency are adjusted at the inverter

47. Inverters and Variable Frequency Drives (VFDs)
Vary frequency to obtain desired speed
Six-step inverter
Receives voltage from the converter
Can control the voltage or the current
Pulse width modulator (PWM)
Receives fixed dc voltage from converter
Short pulses at low speed; long pulses at high speed

48. Electronically Commutated Motors (ECM)
Used on open drive fans less than 1 hp
Armature commutated with permanent magnets
Motors are factory calibrated
Two-piece motor: motor section and controls
Motor can be checked with an ohmmeter
Controls can be checked with a test module
Defective controls can be replaced

49. Cooling Electric Motors
All motors must be cooled
Hermetic compressor motor are cooled by air and refrigerant
Open motors are cooled by air
Open motors must be located where there is a good supply of air
Some very large motors are cooled by water

50. Summary
Motors facilitate the circulation of air, water, refrigerant and other fluids
Some applications require high starting torque
Motor components include the housing, rotor, stator, end bells, bearings, and motor mount

51. Summary (cont’d.) Electricity and magnetism create motor rotation
Motor speed is determined by the number of poles
The start winding has higher resistance than the run winding
Important motor amperage are LRA, FLA, and RLA

52. Summary (cont’d.) Residences are supplied with single-phase power
Some motors are designed to operate at more than one voltage
Split phase motors have a medium amount of starting torque and good running efficiency

53. Summary (cont’d.)
The centrifugal switch opens and closes its contacts depending on the speed of the motor
The current relay opens and closes its contacts depending on the current flow through the run winding

54. Summary (cont’d.)
The potential relay opens and closes its contacts depending on the induced voltage across the start winding
Capacitor start motors use start capacitors to increase the starting torque of the motor

55. Summary (cont’d.)
The start winding and start capacitor are removed from the circuit after the motor starts
Capacitor start, capacitor run motors use both start and run capacitors
Run capacitors help increase the motor’s running efficiency

56. Summary (cont’d.) The PSC motor uses only a run capacitor
The shaded pole motor has very low starting torque
Three-phase motors are used for commercial and industrial applications

57. Summary (cont’d.)
The PTC and NTC are electronic devices that change their resistance as the sensed temperature changes
Variable speed motors ramp up and down, often using dc converters, inverters and rectifiers
ECM motors are commutated with permanent magnets

58. For more information please contact Mark T. Weber At
North Seattle Community College